arXiv subjects

T. Ohashi

Publications and source records attributed to T. Ohashi.

At least 19 recordsLinked to original sources

arXiv2026

Coupled structural and electronic evolution under pressure in CuIr2Se4, CuRh2S4, and CuRh2Se4

Spinel chalcogenides provide a platform for investigating the interplay among metallic, superconducting, and pressure-induced insulating states. Here, we combine synchrotron powder X-ray diffraction and electrical-resistivity measurements to investigate the pressure evolution of CuIr2Se4, CuRh2S4, and CuRh2Se4 over pressure ranges extending beyond those previously explored. High-pressure diffraction reveals closely related monoclinic supercells in all three compounds. For CuIr2Se4 and CuRh2S4, constrained profile fits based on structural models relaxed using density functional theory are compatible with Phase-IV-type bond-disproportionated structures, whereas the data for CuRh2Se4 establish a compatible monoclinic unit cell without resolving its atomic-scale ordering pattern. Insulating-like transport develops abruptly over a narrow pressure range in CuIr2Se4 but more gradually over broader pressure ranges in the Rh-based compounds, in close correspondence with their respective structural transformations. We also establish previously unreported bulk superconductivity in CuIr2Se4 at ambient pressure: zero resistance is attained at 0.29 K, and the accompanying ac diamagnetic response is consistent with nearly complete superconducting shielding. These results establish a close relationship between the formation of the high-pressure monoclinic phases and the evolution toward insulating transport, and demonstrate that transition-metal and chalcogen substitutions tune the characteristic pressure scales and the competition with superconductivity within a closely related structural framework.

cond-mat.mtrl-sci↗

arXiv2025

Reexamination of the charge-ordered dimer pattern in the spinel compound CuIr2S4 using single-crystal synchrotron x-ray diffraction

We have re-investigated the crystal structure of a spinel type CuIr2S4 at low temperatures using a single-crystal in a synchrotron radiation x-ray diffraction experiment. The crystal structure of the low-temperature phase of CuIr2S4 has been already studied by diffraction experiments using a powder sample, and it has been reported that the formation of dimer molecules accompanied by charge ordering of Ir has been achieved. The crystal structure of the low-temperature phase obtained in our reanalysis was the same as the previously reported structure in that it showed the formation of Ir dimers accompanied by charge ordering, but the charge ordering pattern and arrangement of the dimers in the unit cell were different. We will discuss the validity of the structure obtained in this study and provide the structural parameters revealed in the reanalysis. The results of this study should provide a basis for further studies of the physical properties of CuIr2S4, which are still being actively investigated.

cond-mat.str-el↗

arXiv2018

X-ray study of the double radio relic Abell 3376 with Suzaku

We present an X-ray spectral analysis of the nearby double radio relic merging cluster Abell 3376 ($z$ = 0.046), observed with the $Suzaku$ XIS instrument. These deep ($\sim$360 ks) observations cover the entire double relic region in the outskirts of the cluster. These diffuse radio structures are amongst the largest and arc-shaped relics observed in combination with large-scale X-ray shocks in a merging cluster. We confirm the presence of a stronger shock (${\cal M}_{\rm{W}}$ = 2.8 $\pm~0.4$) in the western direction at $r\sim26$', derived from a temperature and surface brightness discontinuity across the radio relic. In the East, we detect a weaker shock (${\cal M}_{\rm{E}}$ = 1.5 $\pm~0.1$) at $r\sim8$', possibly associated to the 'notch' of eastern relic, and a cold front at $r\sim3$'. Based on the shock speed calculated from the Mach numbers, we estimate that the dynamical age of the shock front is $\sim$0.6 Gyr after core passage, indicating that Abell 3376 is still an evolving merging cluster and that the merger is taking place close to the plane of the sky. These results are consistent with simulations and optical and weak lensing studies from the literature.

astro-ph.GA↗

arXiv2017

Resonant scattering in the Perseus Cluster: spectral model for constraining gas motions with Astro-H

X-ray spectra from cores of galaxy clusters can be strongly distorted by resonant scattering of line photons, affecting metal abundance and gas velocity measurements. We introduce simulated spectral models that take into account the resonant scattering effect, radial variations of thermodynamic properties of the hot gas, projection effects and small-scale isotropic gas motions. The key feature of the models is that all these effects are treated self-consistently for the whole spectrum, rather than for individual lines. The model spectra are publicly available and can be used for direct comparison with observed projected spectra. Comparison with the existing XMM-Newton and Chandra data of the Perseus Cluster shows that even though there is no strong evidence for the resonant scattering in Perseus, the low energy resolution of the X-ray CCDs is not sufficient to robustly distinguish spectral distortions due to the resonant scattering, different metal abundance profiles and different levels of gas turbulence. Future Astro-H data will resolve most of the problems we are facing with CCDs. With the help of our models, the resonant scattering analysis can be done self-consistently using the whole spectral information, constraining the level of gas turbulence already with a 100 ks observation with Astro-H.

astro-ph.HE↗

arXiv2016

Suzaku observations of the merging galaxy cluster Abell2255: The northeast radio relic

We present the results of deep 140 ks Suzaku X-ray observations of the north-east (NE) radio relic of the merging galaxy cluster Abell2255. The temperature structure of Abell2255 is measured out to 0.9 times the virial radius (1.9 Mpc) in the NE direction for the first time. The Suzaku temperature map of the central region suggests a complex temperature distribution, which agrees with previous work. Additionally, on a larger-scale, we confirm that the temperature drops from 6 keV around the cluster center to 3 keV at the outskirts, with two discontinuities at {\it r}$\sim$5\arcmin~(450 kpc) and $\sim$12\arcmin~(1100 kpc) from the cluster center. Their locations coincide with surface brightness discontinuities marginally detected in the XMM-Newton image, which indicates the presence of shock structures. From the temperature drop, we estimate the Mach numbers to be ${\cal M}_{\rm inner}\sim$1.2 and, ${\cal M}_{\rm outer}\sim$1.4. The first structure is most likely related to the large cluster core region ($\sim$350--430 kpc), and its Mach number is consistent with the XMM-Newton observation (${\cal M}\sim$1.24: Sakelliou & Ponman 2006). Our detection of the second temperature jump, based on the Suzaku key project observation, shows the presence of a shock structure across the NE radio relic. This indicates a connection between the shock structure and the relativistic electrons that generate radio emission. Across the NE radio relic, however, we find a significantly lower temperature ratio ($T_1/T_2\sim1.44\pm0.16$ corresponds to~${\cal M}_{\rm X-ray}\sim1.4$) than the value expected from radio wavelengths, based on the standard diffusive shock acceleration mechanism ($T_1/T_2>$ 3.2 or ${\cal M}_{\rm Radio}>$ 2.8).

astro-ph.HE↗

arXiv2015

Central Mass Profiles of the Nearby Cool-core Galaxy Clusters Hydra A and A478

We perform a weak-lensing study of the nearby cool-core galaxy clusters, Hydra A ($z=0.0538$) and A478 ($z=0.0881$), of which brightest cluster galaxies (BCGs) host powerful activities of active galactic nuclei (AGNs). For each cluster, the observed tangential shear profile is well described either by a single Navarro--Frenk--White model or a two-component model including the BCG as an unresolved point mass. For A478, we determine the BCG and its host-halo masses from a joint fit to weak-lensing and stellar photometry measurements. We find that the choice of initial mass functions (IMFs) can introduce a factor of two uncertainty in the BCG mass, whereas the BCG host halo mass is well constrained by data. We perform a joint analysis of weak-lensing and stellar kinematics data available for the Hydra A cluster, which allows us to constrain the central mass profile without assuming specific IMFs. We find that the central mass profile ($r<300$kpc) determined from the joint analysis is in excellent agreement with those from independent measurements, including dynamical masses estimated from the cold gas disk component, X-ray hydrostatic total mass estimates, and the central stellar mass estimated based on the Salpeter IMF. The observed dark-matter fraction around the BCG for Hydra A is found to be smaller than those predicted by adiabatic contraction models, suggesting the importance of other physical processes, such as the the AGN feedback and/or dissipationless mergers.

astro-ph.CO↗

arXiv2015

Exploring the bridge between A3556 and A3558 in the Shapley Supercluster

Looking at the region connecting two clusters is a promising way to identify and study the Warm-Hot Intergalactic Medium. Observations show that the spectrum of the bridge between A3556 and A3558 has a stronger soft X-ray emission than the nearby region. Suzaku observations could not discriminate the origin of the extra emission. In this work we analyze a dedicated Chandra observation of the same target to identify point sources and characterize the background emission in the bridge. We find that the count number of the point sources is much higher than average field population (using CDFS~4~Ms as a reference). Moreover, the shape of the cumulative distribution resembles that of galaxy distribution suggesting that the point sources are galaxies in a filament. The Suzaku extra emission is well explained by the high abundance of point sources identified by Chandra. Furthermore, we used optical/IR observations of point sources in the same field to estimate the density of the putative filament as rho~150 rho_b$, below Suzaku sensitivity.

astro-ph.CO↗

arXiv2015

DIOS: the dark baryon exploring mission

DIOS (Diffuse Intergalactic Oxygen Surveyor) is a small satellite aiming for a launch around 2020 with JAXA's Epsilon rocket. Its main aim is a search for warm-hot intergalactic medium with high-resolution X-ray spectroscopy of redshifted emission lines from OVII and OVIII ions. The superior energy resolution of TES microcalorimeters combined with a very wide field of view (30--50 arcmin diameter) will enable us to look into gas dynamics of cosmic plasmas in a wide range of spatial scales from Earth's magnetosphere to unvirialized regions of clusters of galaxies. Mechanical and thermal design of the spacecraft and development of the TES calorimeter system are described. We also consider revising the payload design to optimize the scientific capability allowed by the boundary conditions of the small mission.

astro-ph.IM↗

arXiv2014

ASTRO-H White Paper - Introduction

ASTRO-H White Papers are meant to provide useful information to scientists who plan observations from the satellite. This short paper introduces the 16 ASTRO-H White Papers in addition to general description of the satellite and its new features.

astro-ph.HE↗

arXiv2014

Temperature and Entropy Profiles to the Virial Radius of Abell 1246 Cluster Observed with Suzaku

We report properties of the intracluster medium (ICM) in Abell~1246 to the virial radius ($r_{200}$) and further outside as observed with Suzaku. The ICM emission is clearly detected to $r_{200}$, and we derive profiles of electron temperature, density, entropy, and cluster mass based on the spectral analysis. The temperature shows variation from $\sim 7$ keV at the central region to $\sim 2.5$ keV around $r_{200}$. The total mass in $r_{500}$ is $(4.3 \pm 0.4) \times 10^{14}~M_{\odot}$, assuming hydrostatic equilibrium. At $r>r_{500}$, the hydrostatic mass starts to decline and we, therefore, employ the total mass within $r_{200}$ based on weak-lens mass profile obtained from a sample of lower mass clusters. This yields the gas mass fraction at $r_{200}$ consistent with the cosmic baryon fraction, i.e. $\sim 17$%. The entropy profile indicates a flatter slope than that of the numerical simulation, particularly in $r>r_{500}$. These tendencies are similar to those of other clusters observed with Suzaku. We detect no significant ICM emission outside of $r_{200}$, and $2σ$ upper limits of redshifted OVII and OVIII line intensities are constrained to be less than 2.9 and $5.6\times 10^{-7}$ photons cm$^{-2}$ s$^{-1}$ arcmin$^{-2}$, respectively. The OVII line upper limit indicates $n_{\rm H}< 4.7\times 10^{-5}$ cm$^{-3}$ ($Z/0.2~Z_{\odot}$)$^{-1/2}$ ($L/20~{\rm Mpc}$)$^{-1/2}$, which corresponds to an overdensity, $δ<160$ ($Z/0.2~Z_{\odot}$)$^{-1/2}$ ($L/20~{\rm Mpc}$)$^{-1/2}$.

astro-ph.CO↗

arXiv2014

Universal Profiles of the Intracluster Medium from Suzaku X-Ray and Subaru Weak Lensing Obesrvations

We conduct a joint X-ray and weak-lensing study of four relaxed galaxy clusters (Hydra A, A478, A1689 and A1835) observed by both Suzaku and Subaru out to virial radii, with an aim to understand recently-discovered unexpected feature of the ICM in cluster outskirts. We show that the average hydrostatic-to-lensing total mass ratio for the four clusters decreases from \sim 70% to \sim 40% as the overdensity contrast decreases from 500 to the virial value.The average gas mass fraction from lensing total mass estimates increases with cluster radius and agrees with the cosmic mean baryon fraction within the virial radius, whereas the X-ray-based gas fraction considerably exceeds the cosmic values due to underestimation of the hydrostatic mass. We also develop a new advanced method for determining normalized cluster radial profiles for multiple X-ray observables by simultaneously taking into account both their radial dependence and multivariate scaling relations with weak-lensing masses. Although the four clusters span a range of halo mass, concentration, X-ray luminosity and redshift, we find that the gas entropy, pressure, temperature and density profiles are all remarkably self-similar when scaled with the lensing M_200 mass and r_200 radius.The entropy monotonically increases out to \sim 0.5r_200 following the accretion shock heating model K(r)\propto r^1.1, and flattens at \simgt 0.5r_200.The universality of the scaled entropy profiles indicates that the thermalization mechanism over the entire cluster region (>0.1r_200) is controlled by gravitation in a common to all clusters, although the heating efficiency in the outskirts needs to be modified from the standard law.The bivariate scaling functions of the gas density and temperature reveal that the flattening of the outskirts entropy profile is caused by the steepening of the temperature, rather than the flattening of the gas density.

astro-ph.CO↗

arXiv2014

Development of Multilayer Readout Wiring TES Calorimeter for Future X-ray Missions

We have fabricated multilayer readout wiring Transition Edge Sensors (TES), which enable us to realize both large effective area and high-energy resolution for future X-ray astrophysical missions, such as DIOS. By sandwiching a SiO2 insulation layer between Al superconducting signal and return lines, self/mutual inductances and self fielding of bias leads are expected to be reduced. We fabricated 4x4 and 20x20 TES array on the multilayer wiring and tested their performance. Under the low temperature condition, several pixels in the TES array showed sharp superconducting transitions at around ~300 mK. We also succeeded in detecting X-ray signals from the 4x4 TES, contrary to the previous results of 20x20 TES. We further investigated the reasons for the differences between the 4x4 TES and the 20x20 TES, and present future plans for improving the multilayer TES array fabrication.

physics.ins-det↗

arXiv2013

The Hot and Energetic Universe: The X-ray Integral Field Unit (X-IFU) for Athena+

The Athena+ mission concept is designed to implement the Hot and Energetic Universe science theme submitted to the European Space Agency in response to the call for White Papers for the definition of the L2 and L3 missions of its science program. The Athena+ science payload consists of a large aperture high angular resolution X-ray optics and twelve meters away, two interchangeable focal plane instruments: the X-ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). The X-IFU is a cryogenic X-ray spectrometer, based on a large array of Transition Edge Sensors (TES), offering 2.5 eV spectral resolution, with ~5" pixels, over a field of view of 5 arc minutes in diameter. In this paper, we briefly describe the Athena+ mission concept and the X-IFU performance requirements. We then present the X-IFU detector and readout electronics principles, the current design of the focal plane assembly, the cooling chain and review the global architecture design. Finally, we describe the current performance estimates, in terms of effective area, particle background rejection, count rate capability and velocity measurements. Finally, we emphasize on the latest technology developments concerning TES array fabrication, spectral resolution and readout performance achieved to show that significant progresses are being accomplished towards the demanding X-IFU requirements.

astro-ph.IM↗

arXiv2013

The Hot and Energetic Universe: The evolution of galaxy groups and clusters

Major astrophysical questions related to the formation and evolution of structures, and more specifically of galaxy groups and clusters, will still be open in the coming decade and beyond: what is the interplay of galaxy, supermassive black hole, and intergalactic gas evolution in the most massive objects in the Universe - galaxy groups and clusters? What are the processes driving the evolution of chemical enrichment of the hot diffuse gas in large-scale structures? How and when did the first galaxy groups in the Universe, massive enough to bind more than 10^7 K gas, form? Focussing on the period when groups and clusters assembled (0.5<z<2.5), we show that, due to the continuum and line emission of this hot intergalactic gas at X-ray wavelengths, Athena+, combining high sensitivity with excellent spectral and spatial resolution, will deliver breakthrough observations in view of the aforementioned issues. Indeed, the physical and chemical properties of the hot intra-cluster gas, and their evolution across time, are a key to understand the co-evolution of galaxy and supermassive black hole within their environments.

astro-ph.HE↗

arXiv2011

Properties of the intracluster medium of Abell 3667 observed with Suzaku XIS

We observed the northwest region of the cluster of galaxies A3667 with the Suzaku XIS instrument. The temperature and surface brightness of the intracluster medium were measured up to the virial radius (r200-2.3 Mpc). The radial temperature profile is flatter than the average profile for other clusters until the radius reaches the northwest radio relic. The temperature drops sharply from 5 keV to about 2 keV at the northwest radio relic region. The sharp changes of the temperature can be interpreted as a shock with a Mach number of about 2.5. The entropy slope becomes flatter in the outer region and negative around the radio relic region. In this region, the relaxation timescale of electron-ion Coulomb collisions is longer than the time elapsed after the shock heating and the plasma may be out of equilibrium. Using differential emission measure (DEM) models, we also confirm the multi-temperature structure around the radio relic region, characterized by two peaks at 1 keV and 4 keV. These features suggest that the gas is heated by a shock propagating from the center to outer region.

astro-ph.CO↗

arXiv2011

X-Ray Study of the Outer Region of Abell 2142 with Suzaku

We observed outer regions of a bright cluster of galaxies A2142 with Suzaku. Temperature and brightness structures were measured out to the virial radius ($r_{200}$) with good sensitivity. We confirmed the temperature drop from 9 keV around the cluster center to about 3.5 keV at $r_{200}$, with the density profile well approximated by the $β$ model with $β= 0.85$. Within $0.4\r_{200}$, the entropy profile agrees with $r^{1.1}$, as predicted by the accretion shock model. The entropy slope becomes flatter in the outer region and negative around $r_{200}$. These features suggest that the intracluster medium in the outer region is out of thermal equilibrium. Since the relaxation timescale of electron-ion Coulomb collision is expected to be longer than the elapsed time after shock heating at $r_{200}$, one plausible reason of the low entropy is the low electron temperature compared to that of ions. Other possible explanations would be gas clumpiness, turbulence and bulk motions of ICM\@. We also searched for a warm-hot intergalactic medium around $r_{200}$ and set an upper limit on the oxygen line intensity. Assuming a line-of-sight depth of 2 Mpc and oxygen abundance of 0.1 solar, the upper limit of an overdensity is calculated to be 280 or 380, depending on the foreground assumption.

astro-ph.CO↗

arXiv2011

ORIGIN: Metal Creation and Evolution from the Cosmic Dawn

ORIGIN is a proposal for the M3 mission call of ESA aimed at the study of metal creation from the epoch of cosmic dawn. Using high-spectral resolution in the soft X-ray band, ORIGIN will be able to identify the physical conditions of all abundant elements between C and Ni to red-shifts of z=10, and beyond. The mission will answer questions such as: When were the first metals created? How does the cosmic metal content evolve? Where do most of the metals reside in the Universe? What is the role of metals in structure formation and evolution? To reach out to the early Universe ORIGIN will use Gamma-Ray Bursts (GRBs) to study their local environments in their host galaxies. This requires the capability to slew the satellite in less than a minute to the GRB location. By studying the chemical composition and properties of clusters of galaxies we can extend the range of exploration to lower redshifts (z ~ 0.2). For this task we need a high-resolution spectral imaging instrument with a large field of view. Using the same instrument, we can also study the so far only partially detected baryons in the Warm-Hot Intergalactic Medium (WHIM). The less dense part of the WHIM will be studied using absorption lines at low redshift in the spectra for GRBs.

astro-ph.IM↗

arXiv2011

International X-ray Observatory (IXO) Assessment Study Report for the ESA Cosmic Vision 2015-2025

The International X-Ray Observatory (IXO) will address fundamental questions in astrophysics, including "When did the first SMBH form? How does large scale structure evolve? What happens close to a black hole? What is the connection between these processes? What is the equation of state of matter at supra-nuclear density?" This report presents an overview of the assessment study phase of the IXO candidate ESA L-class Cosmic Vision mission. We provide a description of the IXO science objectives, the mission implementation and the payload. The performance will offer more than an order of magnitude improvement in capability compared with Chandra and XMM-Newton. This observatory-class facility comprises a telescope with highly nested grazing incidence optics with a performance requirement of 2.5 sq.m. of effective area at 1.25 keV with a 5" PSF. There is an instrument complement that provides capabilities in imaging, spatially resolved spectroscopy, timing, polarimetry and high resolution dispersive spectroscopy. Since earlier submissions to the Astro2010 Decadal Survey, substantial technological progress has been made, particularly in the mirror development. Risk reduction measures and important programmatic choices have also been identified. An independent internal Technical and Programmatic Review has also been carried out by ESA, concluding with positive recommendations. Subject to successful conclusion of agreements between the partner space agencies, IXO is fully ready to proceed to further definition, moving towards an eventual launch in 2021-2022.

astro-ph.HE↗